This invention relates to a position detector for detecting the position of a moving object. For example, it is desirable to be able to detect the vertical and rotational position of the rotor blades of a helicopter rotor assembly at various positions along their length as they rotate and to detect the blade pitch angle of each rotor blade so that any lack of uniformity in the deflection, timing or pitch of any or all of the blades in the rotor assembly from any given reference position can be determined and subsequently corrected.
Known techniques for detecting the position of helicopter rotor blades include, for example, the creation (by means of a lens) of an image of a blade tip onto a sensor so that the moving blade will generate a sensor output that can be related to the blade position and the use of sensors and timing marks on the rotor shaft to create timing reference signals which are used to measure any changes in rotational position of the blades.
Another known technique is to use sensors onto which an image of a blade tip is formed onto a single rectangular aperture with a width less than the image width and of sufficient aperture length to accommodate all possible vertical movement of the blade image. Movement of the blade across the aperture changes the amount of light passing through the aperture, such change being detected by an electronic sensor. As the amount of light passing to the sensor when no blade is present can also be measured, a ratio of the two signals can be computed which allows a height signal corresponding to each blade which is independent of the background illumination. However, it is not possible using this technique to detect the pitch of any given blade, and therefore ambiguity of measurement occurs if any given blade is not at the same pitch as the others, and particularly if the rotational position of that same blade is different to the others. In this technique the calibration of the sensor is highly dependent on the amount of light falling on the sensor.
Sensors have also been employed in which an image of a blade tip is formed on a multi element detector (such as a charge coupled device (CCD) as used in a television camera). Accuracy is limited to the image size divided by the number of sensor cells. When a high number of cells is used to increase vertical accuracy, the sensor takes longer to read out the data, thereby reducing the rotational accuracy measurement. Again, it is not possible with this technique to detect the pitch of any given blade and therefore ambiguity of measurement occurs if any given blade is not at the same pitch as the others, particularly if the rotational position of that same blade is different to the others.
Other known techniques include the projection of images of passing blade tips onto linear X-Y graticules as a means of measurement, or onto detector X-Y arrays.
In all of the above mentioned techniques, the measurement relies on a light level contrast between the background (normally the sky in daylight) and the blade. The blade can also be illuminated from some light source and the reflected light measured by the sensor, allowing the position detector to be used in reduced light conditions. In most of the previous methods, in order to relate the amount of light read by the sensor to a given change in position of the blade, the background illumination needs to be measured, and some compensating calculation performed to normalise the data to a known median value.
Again from the prior art, it is also known that precise beams of illumination at a given angular separation can be generated and positioned such that a blade passing through the beams will create pulses of light, the relative timing of which will change with the blade height in some linear relationship to the beam angle, these pulses then being reflected back to the sensor and detected. The accuracy of this method relies on the precise nature of the illuminating beams, as any spreading of the beams (a natural physical phenomenon associated with projected light) will degrade that accuracy.
It is well known by persons skilled in the art of helicopter rotor measurement that in order to correctly adjust the flight characteristics of the main rotor blade in the most effective way for a whole rotation of a rotor system, the position of the blade should be measured shortly after the blade has passed through the “ahead” position over the front of the helicopter. As a first approximation, if the blade is vertical and rotational position is correct at or near this point, then the performance of the blade as it rotates will also be correct at other points on its flight path. For smaller rotors (such as the tail rotor), the measurement should be taken at a point where the blade flight path is not disturbed by air flowing adjacent to the tail boom of the helicopter.
Any sensor therefore needs to view the rotor blades at or near to these positions and in order to do so is often mounted on the external skin of the helicopter with its position set to give an unrestricted field of view of the required part of the blades and therefore the components of the sensor must be capable of withstanding any changes in environmental conditions encountered. Alternately, if internal mounting is possible, with the sensor viewing the blade through the glass canopy of the helicopter, it will need to be placed close to the normal flight instrumentation of the helicopter and therefore be designed so that its operation does not interfere with that instrumentation (for example, the compass, radios and navigation systems). Note, however, that any blade may also be viewed at any other rotational position.
In addition to accurate blade tip tracking, it would be a significant advantage to the helicopter industry to be able to accurately measure the pitch angle of the blade tip and also the vertical position of the blade at other points along its length (the latter being referred to generically in this application as “mid blade tracking”). This is particularly important for some of the larger diameter rotors where, for example, a blade which flies too high in the middle of its length when the tip is flying correctly or too low will be dynamically unstable and will oscillate at some frequency relating to the blade length and stiffness and to rotor speed and such oscillation may cause structural damage to the blade and/or to the entire rotor assembly. It is also of significant importance to be able to measure blade pitch in smaller diameter (often higher speed) tail rotors (and as a further example, for fans, aircraft propellers and similar devices).
Although the invention will be particularly described as applied to fans, propellers and helicopter rotors, it will be appreciated that it can be used for detecting the position of other moving targets or articles, for example the vertical deflection of the end of a moving cantilever beam or components of other rotating machinery.